Methods and apparatuses for modulating threshold voltages of memory cells

ABSTRACT

Methods and apparatuses for increasing the voltage budget window of a memory array are disclosed. One or more pre-bias voltages may be applied across a selected cell by providing voltages to memory access lines coupled to the selected cell. The threshold voltage of the selected cell may decrease responsive to the pre-bias voltage. Conversely, threshold voltage of deselected cells coupled to only one of the memory access lines coupled to the selected cell may increase responsive to the pre-bias voltage. The decrease of the threshold voltage of the selected cell and the increase of the threshold voltage of the deselected cells may increase the voltage window of the memory array.

BACKGROUND

Memory devices may include memory cells for storing data. A memory cellmay include a storage element coupled to a selector device. The storageelement and selector device may be located at an intersection of twomemory access lines (e.g., a word line and a bit line) in a memory arrayhaving a cross-point architecture. The selector may be coupled to a wordline and the storage element may be coupled to a bit line in somearchitectures. The selector device may reduce leakage currents and allowselection of a single storage element for reading and/or writing.

Cross-point memory architecture may have, as selector device, anamorphous chalcogenide element characterized by a threshold voltage(VT). The storage element may also be a chalcogenide device in either acrystalline or amorphous state. The crystalline and amorphous states maycorrespond to different logic states (e.g., ‘1’ or ‘0’). In theamorphous state, the storage element may have the same threshold voltage(VT) as the selector device in some architectures. In otherarchitectures, in the crystalline state, the storage element may havethe same threshold voltage (VT) as the selector device. The overallcross-point memory cell is the series of the selector device and storageelement. The memory cell may have either a low VT (VT_SET) when thestorage element is in a crystalline state or a high VT (VT_RESET) whenthe storage element is in an amorphous state. The high and low thresholdvoltages which may be associated with the amorphous and crystallinestates, respectively, may be representative of different logic statesstored in the memory cell.

A memory array may have a memory voltage window budget that defines themaximum and minimum voltages for selecting memory cells within thememory array. Memory voltage window budget parameters may include themaximum VT_RESET voltage (VT_RESET_max) and the minimum VT_SET voltage(VT_SET_min) of the complete memory array of memory cells. At a firstorder, VT_RESET_max −VT_SET_min should be smaller than half of themaximum operating voltage available in order to properly accessindividual memory cells. VT_RESET_max and VT_SET_min may depend on oneor more physical parameters of the selector device and/or the storageelement (e.g., the chalcogenide chemical composition, thickness,cell-to-cell variability, etc.).

The cell-to-cell VT variability may increase the difference (delta)between VT_RESET_max and VT_SET_min such that the difference betweenVT_RESET_max and VT_SET_min is greater than the available voltagewindow. This may prevent the memory array from meeting the voltagewindow requirement. Currently, techniques to reduce cell-to-cellvariability rely on the reduction of the variability of the physicalparameters of the selector device and storage element that affectthreshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a portion of a memory arrayaccording to an embodiment of the disclosure.

FIG. 2 is a plot of threshold voltage versus time of a memory cellaccording to an embodiment of the disclosure.

FIG. 3 is a plot of voltages provided to memory access lines versus timeaccording to an embodiment of the disclosure.

FIG. 4 is a flow chart of a method according to an embodiment of thedisclosure.

FIG. 5 is a functional diagram of a memory according to an embodiment ofthe disclosure.

FIG. 6 is a schematic illustration of a portion of a memory arrayaccording to an embodiment of the disclosure.

FIG. 7 is a schematic illustration of a portion of a memory arrayaccording to an embodiment of the disclosure.

DETAILED DESCRIPTION

Certain details are set forth below to provide a sufficientunderstanding of embodiments of the invention. However, it will be clearto one skilled in the art that embodiments of the invention may bepracticed without these particular details. Moreover, the particularembodiments of the present invention described herein are provided byway of example and should not be used to limit the scope of theinvention to these particular embodiments. In other instances,well-known circuits, control signals, timing protocols, and softwareoperations have not been shown in detail in order to avoid unnecessarilyobscuring the invention.

As described herein, the threshold voltage (VT) of a memory cell may bemodulated by applying a bias voltage below the VT across the memorycell. Depending, at least in part, on the magnitude of the applied biasvoltage, the VT of the memory cell may either increase or decrease. Ingeneral, the VT of a memory cell may increase responsive to a biasvoltage well below the VT (e.g., bias voltage=3.0V, VT=6.0V).Conversely, the VT of the memory cell may decrease responsive to a biasvoltage below but near the VT (e.g., bias voltage=5.5V, VT=6.0V).Modulating the VT of the memory cell by applying the bias voltage mayallow the voltage window of a memory array to be increased, as well asmeeting voltage window requirements.

In some embodiments described herein, in a memory having a cross-pointmemory architecture, deselected cells may have, at consecutive times, abias voltage applied which may increase the VT of the deselected cells.A memory cell that has been selected to be written to (e.g., targetcell) may be subjected to, just before the selection, a bias voltagenear the selected memory cell's VT, which may decrease the voltagerequired to threshold the memory cell (e.g., the resistance across thememory cell decreases rapidly). This may facilitate the selection of areset memory cell (e.g., relatively high VT) in a memory array whileinhibiting unintended selection of deselected set memory cells (e.g.,relatively low VT).

FIG. 1 is a schematic illustration of a portion of a cross-point memoryarray 100 according to an embodiment of the disclosure. From a voltagewindow budget perspective, the operation that may require the largestvoltage magnitude is the selection of a reset memory cell. As previouslydescribed, a reset memory cell has the chalcogenide of the storageelement in an amorphous state (e.g., reset state). A memory cell in areset state may have the highest threshold voltage (VT_RESET_max) of thememory cells of the memory array.

As shown in FIG. 1, the selected reset cell is labeled target cell T.The target cell T may be coupled to a first memory access line (SELBL)and a second memory access line (SELWL). The deselected cells arelabeled A cells on memory access line SELWL and B cells on memory accessline SELBL. The remaining cells on deselected memory access lines DESBLand DESWL are labeled C cells. In some embodiments, deselected memoryaccess lines DESWL and DESBL may be coupled to a common voltage (e.g.,0V, ground) during a memory operation on target cell T.

To select the target cell T, voltages may be provided to the firstmemory access line SELBL coupled to the target cell T and the secondmemory access SELWL line coupled to the target cell T to apply a biasvoltage across the target cell T. In some embodiments, voltages of equalmagnitude and opposite polarity may be provided to the first memoryaccess line and second memory access line side (e.g., V_SELWL=−V_SELBL).The maximum threshold voltage of the target cell T should be equal to orless than twice the voltage applied to the memory access lines (e.g.,VT_RESET< 2V_SELWL or 2V_SELBL), otherwise, the target cell T may not besuccessfully selected. The provided voltages may be as high as theoff-chip maximum voltage available in order to facilitate selecting thetarget cell T that may have a high reset VT. However, the voltagesprovided to the memory access lines should be low enough to prevent theundesired selection of deselected memory cells. That is, the voltageprovided to the first memory access line SELWL should be less than theminimum threshold voltage of the A cells, and the voltage provided tomemory access line SELBL should be less than the minimum thresholdvoltage of the B cells.

One or more of the deselected memory cells may have a low VT. Forexample, a memory cell where a chalcogenide storage element is in acrystalline state (e.g., a set memory cell). Memory cells with low VTmay be prone to undesired selection during a write operation of theselected memory cell (e.g., target cell T). Undesired selection of adeselected memory cell (e.g., A cells and B cells) may lead tocorruption of the data stored by the deselected memory cell and/ordesired data not being written to the target cell T. To increase thevoltage window, the VT of the deselected cells may be increased, and/orthe VT of the target cell may be decreased.

In some embodiments, a pre-bias voltage may be applied to a memory cell.A pre-bias voltage may be a bias voltage applied to a memory cell priorto a memory operation (e.g., write, read). In some embodiments, thepre-bias voltage may be applied in response to receiving a request for amemory operation. In some embodiments, the memory operation may apply aseparate bias voltage across the memory cell to execute the desiredoperation after the pre-bias voltage has been applied. In someembodiments, one or more pre-bias voltages may be used in place of theseparate bias voltage associated with the memory operation. In someembodiments, the bias voltage associated with the memory operation mayhave a different magnitude and/or duration than the pre-bias voltage. Insome embodiments, the bias voltage associated with the memory operationmay have the same magnitude and/or duration as the pre-bias voltage.

In some embodiments, a low pre-bias voltage (VL) may be applied across amemory cell that is lower than the VT of the memory cell. The lowpre-bias voltage may be such that the ratio of VL/VT is equal to orbelow a threshold value. In some embodiments, the threshold value may be0.8. The VT of the memory cell may increase responsive to applying thelow pre-bias voltage over a period of time. In some embodiments, a highpre-bias voltage (VH) may be applied across a memory cell. The magnitudeof the high pre-bias voltage may be greater than the magnitude of thelow pre-bias voltage (VL<VH). The magnitude of the high pre-bias voltagemay be lower than the VT of the memory cell. However, in contrast toapplying the low pre-bias voltage, the VT of the memory cell maydecrease responsive to applying the high pre-bias voltage for a periodof time. The high pre-bias voltage may be such that the ratio of VH/VTis above a threshold value. In some embodiments, the threshold value maybe 0.8. In summary, when a pre-bias voltage below VT is applied to amemory cell, the VT of the memory cell will increase if the pre-biasvoltage applied is below a threshold value, and the VT of the memorycell will decrease if the pre-bias voltage applied is above thethreshold value. In some embodiments, values of VH and VL may be chosenbased, at least in part, on a typical (e.g., average) VT of a memorycell in a reset state in a memory array. In some embodiments, values ofVH and VL may be chosen based, at least in part, on distributions of oneor more cells in a set state and one or more cells in a reset statewithin an array.

In some embodiments, the change in VT responsive to the applied pre-biasvoltage (VL or VH) may be transient. That is, after a period of time,the VT of the memory cell returns to its original state after thepre-bias voltage is removed. The logic state of the memory cell may notbe altered or permanently altered by applying the pre-bias voltage.

FIG. 2 is a plot 200 of threshold voltage (VT) of a memory cell overtime when a pre-bias voltage (V) is applied across the memory cellaccording to an embodiment of the disclosure. Curves 205 and 210illustrate the rise in VT of a memory cell when a pre-bias voltage Vapplied is a low pre-bias voltage. Curve 210 shows a rise in VT when thelow pre-bias voltage is well below the initial VT of the memory cell(e.g., V=3.0V, VT=6.0V). Curve 205 shows a rise in VT when the lowpre-bias voltage V is closer to the initial VT of the memory cell (e.g.,V=4.0V, VT=6.0V). As shown in FIG. 2, curve 210 has a lower slope thancurve 205. That is, the VT of the memory cell is increasing faster overtime for curve 205 than curve 210. As the pre-bias voltage V increases,the slope of the VT vs. time curve may increase.

However, once the ratio V/VT reaches a threshold value, the slope of theVT vs. time curve may no longer increase and may become negative. Thisnegative slope is shown in curve 215. Curve 215 shows a decrease in VTof a memory cell when the pre-bias voltage V applied is a high pre-biasvoltage (e.g., V=5.5V, VT=6.0V). The bias voltage V may be close inmagnitude to the initial VT such that V/VT is over a threshold value(e.g., V/VT=0.8, V/VT=0.85, V/VT=0.9).

Without being bound to a particular theory, the increase in VT of amemory cell in response to the application of a low pre-bias voltage maybe due to the acceleration of the trapping in a chalcogenide materialwhich may drive the VT increase. Without being bound to a particulartheory, the decrease in VT of a memory cell in response to a highpre-bias voltage may be driven by the current noise around the thresholdvoltage VT which may then trigger a threshold event (e.g., snap) at alower voltage magnitude.

The variations in the rate of change of the VT of a memory cell based onthe magnitude of the pre-bias voltage, as shown by the slopes of the VTvs. time curves 205, 210, and 215, may be used to facilitate expandingthe voltage window of a memory array. In some embodiments, the VTincrease of a memory cell through application of a low pre-bias may beexploited to increase the VT for set memory cells having very low VT,which may provide the advantage of reducing the likelihood of unintendedselection of set memory cells, while the VT decrease of a memory cellthrough application of a high pre-bias may be exploited to lower the VTfor reset memory cells having very high VT, which may facilitateselection of reset memory cells.

By memory cells having very low or very high VT, it is meant to refer tomemory cells that have threshold voltage values that may fall outside anexpected range of VT for a memory array due to cell-to-cell variability.These memory cells may not be able to be properly selected for writingto by conventional selection methods. With conventional methods thesevery low and/or very high VT memory cells may cause data corruption,writing errors, and/or read errors of the memory cells and/or othermemory cells in the memory array that may not be very high or very lowVT cells. Widening the voltage window using pre-bias as described in theembodiments disclosed herein may facilitate operation of memory arraysthat include very high and/or very low VT cells.

FIG. 3 is a plot 300 of pre-bias voltages applied across memory cellsover time according to an exemplary implementation of the disclosure. Inthe exemplary implementation shown in FIG. 3, all deselected memoryaccess lines (e.g., DESBL and DESWL in FIG. 1) may be coupled to acommon voltage (e.g., ground). The operation of the exemplaryimplementation may be a write operation. The memory access lines coupledto a target cell (e.g., SELWL and SELBL in FIG. 1) may be providedvarying voltages over time. Curve 305 shows the voltages applied to afirst memory access line (SELWL) and curve 310 shows the voltagesapplied to a second memory access line (SELBL). As shown in FIG. 3, thefirst and second memory access lines are stepped through a plurality ofvoltages, where the magnitudes of the voltages are increased with eachsubsequent step. For example, the voltage applied to SELWL becomes morenegative and the voltage applied to SELBL becomes more positive. Eachvoltage is held constant for a period of time prior to stepping to thenext voltage. As the magnitude of the voltage of SELWL is increased, theVT of the memory cells coupled to SELWL may increase (e.g., A cellsshown in FIG. 1). As the magnitude of the voltage of SELBL is increased,the VT of the memory cells coupled to SELBL may increase (e.g., B cellsshown in FIG. 1). The magnitude of the pre-bias voltage V across atarget cell coupled to SELWL and SELBL (e.g., target cell T shown inFIG. 1) may be such that the ratio of V/VT of the target cell is above athreshold value. The VT of the target memory cell may decreaseresponsive to the pre-bias voltage. In some embodiments, the pre-biasvoltage across the target cell may be held at this magnitude until theVT of the target cell decreases to where a threshold event 315 occurs atthe target cell. That is, the VT of the target cell decreases until itreaches the magnitude of the pre-bias voltage applied across the targetcell, and the resistance across the target cell decreases, facilitatingcurrent flow through the target cell. As mentioned previously, this maybe referred to as a snap event. In some embodiments, after the finalpre-bias voltage has been held for a period of time, a subsequent biasvoltage associated with the memory operation (e.g., write) may beapplied across the target cell to threshold the target cell. In someembodiments, the target cell may have a low VT (e.g., set state). Whenthe target cell has a low VT, it may snap at a lower pre-bias voltagestep than a target cell having a high VT. Once the target cell hassnapped, the magnitude of the voltage of the memory access lines maycease to be increased. In other words, in some embodiments, fewerpre-bias voltage steps may be applied to a target cell, based, at leastin part, on the VT of the target cell.

In one illustrative example, SELWL and SELBL may both initially beprovided 0V. Subsequently, SELWL may be biased to −1.5V and SELBL may bebiased to 1.5V. After a period of time, SELWL may be biased to −3.0V andSELBL may be biased to 3.0V. The SELWL may then be biased to −5.0V andSELBL may then be biased to 5.0V. As shown in FIG. 3, SELWL and SELBLmay be held at these voltage levels until a snap event occurs at thememory cell coupled between SELWL and SELBL. Other voltage magnitudesmay be used for the voltages in some embodiments. The voltages providedto the memory access lines to provide pre-bias voltage levels appliedacross the memory cell may be selected based, at least in part, on athreshold value of a ratio V/VT (e.g., 0.8).

In some embodiments, more or fewer pre-bias voltage steps may be appliedacross a memory cell. As discussed previously, more or fewer pre-biasvoltage steps may be applied in addition to or in lieu of a bias voltageassociated with a memory operation. In some embodiments, the SELWL andSELBL are not provided voltages equal in magnitude. For example, SELWLmay be biased to −1.0V and SELBL may be biased to −1.5V. In someembodiments, SELWL and SELBL are stepped sequentially, where SELWL isstepped to the next voltage prior to SELBL being stepped to the nextvoltage, or vice versa. Although shown as square steps in FIG. 3, insome embodiments, the pre-bias voltages may be gradually ramped betweeneach pre-bias voltage to be held for a period of time. In someembodiments, the voltage differences between adjacent steps may beequal. In some embodiments, there may be a defined number of pre-biasedvoltages applied. In some embodiments, the pre-bias voltages may bestepped until a threshold event occurs at a target cell. Other protocolsfor increasing the pre-bias voltage across the target cell may also beused.

In another embodiment similar to the one shown in FIG. 3, SELWL mayinitially be provided +4V and SELBL may be provided −4V. Deselectedcells coupled to only one or the other of the two memory access linesSELWL, SELBL may have a magnitude of 4V across the cells. One or moredeselected cells may be in a set state. The one or more deselected cellsin the set state may have a distribution of threshold voltages VTcentered at 5V. One or more deselected cells may be in a reset state.The one or more deselected cells in the reset state may have adistribution of threshold voltages VT centered at 9V. Both reset and setstate deselected cells are biased at a low pre-bias voltage, and the VTof both reset and set state deselected cells may increase.

A targeted cell coupled to both memory access lines SELWL, SELBL may behave a voltage magnitude of 8V across the cell. If the target cell is ina reset state, the target cell may have a VT at or near 9V. The voltageapplied across the target cell by the memory access lines may be a highpre-bias voltage and may cause the VT of the target cell to decrease.Alternatively, if the target cell is in a set state, the target cell mayhave a VT at or near 5V, and the target cell may experience a thresholdevent.

If the target cell is in the reset state and has a VT over 8V even afterthe VT has decreased responsive to the high pre-bias voltage, the SELWLmay subsequently be provided a voltage of +5V and the SELBL provided avoltage of −5V. The deselected cells in either set or reset states maycontinue to have low pre-bias voltages applied across the cells. Thedistribution of threshold voltages for deselected cells in both statesmay continue to increase. The target cell may have a voltage magnitudeof 10V applied across the cell. The applied voltage may be above the VTof the target cell and cause the target cell to threshold.

As described in this exemplary embodiment, the initial biasing of thememory access lines may reduce the threshold voltage of a reset cell ata high end of the distribution of threshold voltages (e.g., above 10V)to be selected by applying 10V across the target cell by initiallypre-biasing the target cell to reduce the threshold voltage of thetarget cell to below 10V. The particular voltages have been provided byway of example, and other voltages, voltage distributions, and/or numberof pre-bias voltages may be used in other embodiments.

FIG. 4 is a flow chart of a method 400 according to an embodiment of thedisclosure. In some embodiments, method 400 may be used to select atarget cell of a memory array coupled to two memory access lines. Themethod 400 may reduce the VT of the target cell and/or increase the VTof deselected memory cells in the memory array that are coupled to oneor the other of the two memory access lines. The method 400 may increasethe voltage window of the memory array in some embodiments. In Step 405,a first pre-bias voltage may be applied across a target cell. The firstpre-bias voltage may be applied across the memory cell by providingvoltages to memory access lines as described in reference to FIG. 3. Thefirst pre-bias voltage may be held constant across the target cell atStep 410 for a period of time. As described above, during this time,deselected memory cells coupled to one of the memory access lines mayhave an increase in VT in response to the first pre-bias voltage. AtStep 415, a second pre-bias voltage may be applied across the targetcell. The second pre-bias voltage may be greater in magnitude than thefirst pre-bias voltage. In some embodiments, the first pre-bias voltageis a low pre-bias voltage (VL) and the second pre-bias voltage is a highpre-bias voltage (VH). The magnitude of the first pre-bias voltage maybe such that the ratio VL/VT is less than a threshold value of thetarget cell. The magnitude of the second pre-bias voltage may be suchthat the ratio VH/VT is above a threshold value of the target cell. Aswill be described in more detail below, pre-bias voltages may be appliedacross deselected cells coupled to one of the two memory access lineswhen the first and second pre-bias voltages are applied across thetarget cell. The pre-bias voltages applied across the deselected cellsmay be low pre-bias voltages (VL) having magnitudes such that the ratioVL/VT is less than a threshold value of the deselected cells.

The second pre-bias voltage may be held constant across the target cellat Step 420 for a period of time. The target cell may experience adecrease in VT responsive to the second pre-bias voltage. As mentionedpreviously, the second pre-bias voltage may have a magnitude such thatthe ratio of the pre-bias voltage to the VT of the target cell isgreater than a threshold value (e.g., 0.8). In contrast, deselectedmemory cells coupled to one of the memory access lines further coupledto the target cell may further increase VT in response to the secondpre-bias voltage as the deselected memory cells may be biased by onehalf of the second pre-biased voltage (e.g., VH/2). One half of thesecond pre-biased voltage may be such that the ratio over VT is lessthan the threshold value (e.g., (VH/2)/VT<0.8) of deselected memorycells. In some embodiments, the second pre-bias voltage may continue tobe held across the target cell until the target cell thresholds (e.g.,snaps back) at Step 425, at which time the target cell becomesconductive and allows current to flow. Alternatively, a separate biasvoltage associated with a memory operation may be applied to the targetcell at Step 430. The bias voltage applied at Step 430 may induce athreshold event in the target cell and/or other effect desired for thememory operation. Although two pre-bias voltages are applied in method400, more than two pre-biasing voltages may be applied across the targetcell (e.g., a third pre-bias voltage, a fourth pre-bias voltage, and soon).

The pre-bias voltages may be applied across the memory cell by memoryaccess lines responsive to one or more control signals. The memoryaccess lines may be coupled to memory access circuits (e.g., column androw decoder circuits) which may be further coupled to control logic,which may be configured to provide internal control signals to memorycircuits (e.g., memory access circuits) to implement the method 400. Thecontrol logic may be implemented as hardware, software, or combinationsthereof. For example, in some embodiments, the control logic may be anintegrated circuit including circuits such as logic circuits andcomputational circuits. The circuits of the control logic may operate toexecute various operations and provide control signals to other circuitsof a memory array, such as memory array 100. In some embodiments, thecontrol logic may be implemented as multiple control logic circuits.Other control logic circuits and/or other apparatuses may be used forimplementing method 400.

FIG. 5 illustrates an apparatus that includes a memory device 500according to an embodiment of the disclosure. The memory device 500includes a memory array 560 with a plurality of memory cells that areconfigured to store data. The memory cells may be accessed in the arraythrough the use of various signal lines, word lines (WLs) and/or bitlines (BLs). The memory cells may be non-volatile memory cells, such asNAND or NOR flash cells, phase change memory cells, or may generally beany type of memory cells. The memory cells of the memory array 560 canbe arranged in a memory array architecture. For example, in oneembodiment, the memory cells are arranged in a 3D cross-pointarchitecture. In other embodiments, other memory array architectures maybe used, for example, a single-level cross-point architecture, amongothers. The memory cells may be single level cells configured to storedata for one bit of data. The memory cells may also be multi-level cellsconfigured to store data for more than one bit of data.

A data strobe signal DQS may be transmitted through a data strobe bus(not shown). The DQS signal may be used to provide timing informationfor the transfer of data to the memory device 500 or from the memorydevice 504. The I/O bus 528 is connected to an I/O control circuit 520that routes data signals, address information signals, and other signalsbetween the I/O bus 528 and an internal data bus 522, an internaladdress bus 524, and/or an internal command bus 526. An address register525 may be provided address information by the I/O control circuit 520to be temporarily stored. The I/O control circuit 520 is coupled to astatus register 534 through a status register bus 532. Status bitsstored by the status register 534 may be provided by the I/O controlcircuit 520 responsive to a read status command provided to the memorydevice 500. The status bits may have respective values to indicate astatus condition of various aspects of the memory and its operation.

The memory device 500 also includes a control logic 510 that receives anumber of control signals 538 either externally or through the commandbus 526 to control the operation of the memory device 500. The controlsignals 538 may be implemented with any appropriate interface protocol.For example, the control signals 538 may be pin based, as is common indynamic random access memory and flash memory (e.g., NAND flash), orop-code based. Example control signals 538 include clock signals,read/write signals, clock enable signals, etc. A command register 536 iscoupled to the internal command bus 526 to store information received bythe I/O control circuit 520 and provide the information to the controllogic 510. The control logic 510 may further access a status register534 through the status register bus 532, for example, to update thestatus bits as status conditions change. The control logic 510 may beconfigured to provide internal control signals to various circuits ofthe memory device 500. For example, responsive to receiving a memoryaccess command (e.g., read, write), the control logic 510 may provideinternal control signals to control various memory access circuits toperform a memory access operation. The various memory access circuitsare used during the memory access operation, and may generally includecircuits such as row and column decoders, charge pump circuits, signalline drivers, data and cache registers, I/O circuits, as well as others.

The address register 525 provides block-row address signals to a rowdecoder 540 and column address signals to a column decoder 550. The rowdecoder 540 and column decoder 550 may be used to select blocks ofmemory cells for memory operations, for example, read and writeoperations. The row decoder 540 and/or the column decoder 550 mayinclude one or more signal line drivers configured to provide a biasingsignal to one or more of the signal lines in the memory array 560. Insome embodiments, the signal line drivers of the row decoder 540 and/orthe column decoder 550 may provide one or more pre-biasing signals toone or more signal lines in the memory array 560 to apply one or morepre-bias voltages across a memory cell of the memory array 560. Thepre-bias voltages may be applied responsive to one or more controlsignals provided by the control logic 510. The control logic 510 may beused to implement the control logic as previously described in referenceto FIG. 4.

For a write operation, after the row address signals have been appliedto the address bus 524, the I/O control circuit 520 routes write datasignals to a cache register 570. The write data signals are stored inthe cache register 570 in successive sets each having a sizecorresponding to the width of the I/O bus 528. The cache register 570sequentially stores the sets of write data signals for an entire row orpage of memory cells in the array 560. All of the stored write datasignals are then used to write a row or page of memory cells in thearray 560 selected by the block-row address coupled through the addressbus 524. In a similar manner, during a read operation, data signals froma row or block of memory cells selected by the block-row address coupledthrough the address bus 524 are stored in a data register 580. The dataregister 580 and the cache register 570 may act as a single register forsome page operations. For example, data stored in the data register 580may be also stored in the cache register 570. Sets of data signalscorresponding in size to the width of the I/O bus 528 are thensequentially transferred through the I/O control circuit 520 from thedata register 580 and/or the cache register 570 to the I/O bus 528.

FIG. 6 is a diagram illustrating a portion of an array 600 of memorycells according to an embodiment of the disclosure. The array 600 may beused to implement the memory array 560 of FIG. 5 in some embodiments. Inthe example illustrated in FIG. 6, the array 600 is a cross-point arrayincluding a first number of conductive lines 630-0, 630-1, . . . ,630-N, e.g., access lines, which may be referred to herein as wordlines, and a second number of conductive lines 620-0, 620-1, . . . ,620-M, e.g., access lines, which may be referred to herein as bit lines.A memory cell 625 is located at each of the intersections of the wordlines 630-0, 630-1, . . . , 630-N and bit lines 620-0, 620-1, . . . ,620-M and the memory cells 625 can function in a two-terminalarchitecture, e.g., with a particular word line 630-0, 630-1, . . . ,630-N and bit line 620-0, 620-1, . . . , 620-M serving as the electrodesfor the memory cells 625.

The memory cells 625 can be resistance variable memory cells, e.g., RRAMcells, CBRAM cells, PCRAM cells, and/or STT-RAM cells, among other typesof memory cells. Memory cells can include, for example, a storageelement, e.g., memory material, and a select element, e.g., a selectdevice, an access device. The select element can be a diode or anon-ohmic device (NOD), among others. The select element can include, insome examples, select material, a first electrode material, and a secondelectrode material. The storage element of memory cell 625 can include amemory portion of the memory cell 625, e.g., the portion programmable todifferent data states. For instance, in resistance variable memorycells, a storage element can include the portion of the memory cellhaving a resistance that is programmable to particular levelscorresponding to particular data states responsive to appliedprogramming voltage and/or current pulses, for instance. Embodiments arenot limited to a particular resistance variable material or materialsassociated with the storage elements of the memory cells 625. The memorycell 625 can include a material programmable to different data states(e.g., chalcogenide). For instance, the memory cell 625 may be writtento store particular levels corresponding to particular data statesresponsive to applied writing voltage and/or current pulses, forinstance. Embodiments are not limited to a particular material ormaterials. For instance, the material can be a chalcogenide formed ofvarious doped or undoped materials. Other examples of materials that canbe used to form storage elements include binary metal oxide materials,colossal magnetoresistive materials, and/or various polymer basedresistance variable materials, among others.

In operation, the memory cells 625 of array 600 can be written to byapplying a voltage, e.g., a write voltage, across the memory cells 625via selected word lines 630-0, 630-1, . . . , 630-N and bit lines 620-0,620-1, . . . , 620-M. A sensing, e.g., read, operation can be used todetermine the data state of a memory cell 625 by sensing current, forexample, on a bit line 620-0, 620-1, . . . , 620-M corresponding to therespective memory cell responsive to a particular voltage applied to theselected word line 630-0, 630-1, . . . , 630-N to which the respectivecell is coupled.

FIG. 7 is a diagram illustrating a portion of an array 700 of memorycells. The array 700 may be used to implement the memory array 560 ofFIG. 5 in some embodiments. In the example illustrated in FIG. 7, thearray 700 is configured in a cross-point memory array architecture,e.g., a three-dimensional (3D) cross-point memory array architecture.The multi-deck cross-point memory array 700 comprises a number ofsuccessive memory cells, e.g., 705, 715, 725 disposed betweenalternating, e.g., interleaved, decks of word lines, e.g., 730-0, 730-1,. . . , 730-N and 712-0, 712-1, . . . , 712-N extending in a firstdirection and bit lines, e.g., 720-0, 720-1, . . . , 720-M and 714-0,1314-1, . . . , 714-M extending in a second direction. The number ofdecks can be expanded in number or can be reduced in number, forexample. Each of the memory cells 705, 715, 725 can be configuredbetween word lines, e.g., 730-0, 730-1, . . . , 730-N and 712-0, 712-1,. . . , 712-N and bit lines, e.g., 720-0, 720-1, . . . , 720-M and714-0, 714-1, . . . , 714-M, such that a single memory cell 705, 715,725 is directly electrically coupled with and is electrically in serieswith its respective bit line and word line. For example, array 700 caninclude a three-dimensional matrix of individually-addressable, e.g.,randomly accessible, memory cells that can be accessed for dataoperations, e.g., sense and write, at a granularity as small as a singlestorage element or multiple storage elements. In a number ofembodiments, memory array 700 can include more or less bit lines, wordlines, and/or memory cells than shown in the examples in FIG. 7.

Memories in accordance with embodiments of the present invention may beused in any of a variety of electronic devices including, but notlimited to, computing systems, electronic storage systems, cameras,phones, wireless devices, displays, chip sets, set top boxes, or gamingsystems.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

What is claimed is:
 1. An apparatus comprising: a first memory accessline; a second memory access line; a memory cell coupled to the firstmemory access line and to the second memory access line, wherein thememory cell is configured to have a threshold voltage; memory accesscircuits coupled to the first and second memory access lines andconfigured to provide a respective voltage to the first and secondmemory access lines; and a control logic circuit coupled to the memoryaccess circuits and configured to provide control signals to memoryaccess circuits to apply a first pre-bias voltage across the memory cellby the first and second memory access lines and to further applythereafter a second pre-bias voltage across the memory cell by the firstand second memory access lines, wherein the second pre-bias voltage hasa greater magnitude than the first pre-bias voltage, the first andsecond pre-bias voltages are below the threshold voltage, and wherein aratio of the second pre-bias voltage and the threshold voltage are abovea threshold value.
 2. The apparatus of claim 1, wherein the thresholdvoltage of the memory cell decreases responsive to the second pre-biasvoltage.
 3. The apparatus of claim 1, wherein the memory cell includes aselector device and a storage element.
 4. The apparatus of claim 3,wherein the storage element has a first threshold voltage correspondingto a first logic state or a second threshold voltage corresponding to asecond logic state.
 5. The apparatus of claim 4, wherein the storageelement includes a chalcogenide material.
 6. The apparatus of claim 1,wherein the first and second memory access lines are configured toreceive voltages of equal magnitude.
 7. The apparatus of claim 1,further comprising a memory array, the memory array comprising: aplurality of memory access lines; and a plurality of memory cellscoupled to respective ones of the plurality of memory access lines,wherein the first and second memory access lines are each one of theplurality of memory access lines and the memory cell is one of theplurality of memory cells.
 8. The apparatus of claim 7, wherein thememory array is a three-dimensional array.
 9. The apparatus of claim 7,wherein the plurality of memory access lines and the plurality of memorycells are arranged in a cross-point architecture.
 10. The apparatus ofclaim 1, wherein the second pre-bias voltage is configured to thresholdthe memory cell.
 11. The apparatus of claim 1, wherein the control logicis further configured to provide control signals to memory accesscircuits to apply a bias voltage across the memory cell by the first andsecond memory access lines.
 12. The apparatus of claim 1, furthercomprising a deselected memory cell coupled to the first or secondmemory access line, wherein a third pre-bias voltage is applied acrossthe deselected cell responsive to the first or the second pre-biasvoltage being applied across the memory cell, wherein a ratio of thethird pre-bias voltage and a threshold voltage of the deselected memorycell are below a threshold value.
 13. A method comprising: applying afirst pre-bias voltage across a memory cell; holding the first pre-biasvoltage across the memory cell for a first period of time; applying asecond pre-bias voltage across the memory cell; and holding the secondpre-bias voltage across the memory cell for a second period of time;wherein the second pre-bias voltage has a magnitude such that a ratio ofthe second pre-bias voltage and a threshold voltage of the memory cellis above a threshold value.
 14. The method of claim 13, furthercomprising holding the second pre-bias voltage across the memory celluntil the memory cell thresholds.
 15. The method of claim 13, furthercomprising applying a bias voltage across the memory cell.
 16. Themethod of claim 13, wherein the magnitude of the first pre-bias voltageis less than the magnitude of the second pre-bias voltage.
 17. Themethod of claim 13, wherein the first period of time is less than thesecond period of time.
 18. A method, comprising: applying a firstvoltage to a first access line, wherein threshold voltages of a firstplurality of memory cells coupled to the first access line areincreased; applying a second voltage to a second access line, whereinthreshold voltages of a second plurality of memory cells coupled to thesecond access line are increased; and applying a sum of the first andsecond voltages across a target memory cell coupled to both the firstand second access lines, wherein the threshold voltage of the targetmemory cell are decreased.
 19. The method of claim 18, furthercomprising: increasing the magnitude of the first and second voltagesover time.
 20. The method of claim 19 wherein increasing the magnitudeof the first and second voltages over time comprises stepping the firstand second voltages through a plurality of increasing voltagemagnitudes.
 21. The method of claim 18, wherein applying the firstvoltage to the first access line comprises applying a voltage, wherein aratio of the voltage with the threshold voltages of the first pluralityof memory cells is equal to or less than 0.8.
 22. The method of claim 18wherein applying the sum of the first and second voltages across atarget memory cell comprises applying a sum voltage, wherein a ratio ofthe sum voltage with the threshold voltage of the target memory cellthat is greater than 0.8.